Aerosolized Fosfomycin/Aminoglycoside Combination for the Treatment of Bacterial Respiratory Infections

ABSTRACT

A fosfomycin plus tobramycin combination formulation for delivery by aerosolization is described. The concentrated fosfomycin tobramycin combination formulation containing an efficacious amount of fosfomycin plus tobramycin is able to inhibit susceptible bacteria. Fosfomycin and tobramycin are formulated separately in a dual ampoule such that when reconstituted, the pH is between 4.5 and 8.0 or as a dry powder. The method for treatment of respiratory tract infections by a formulation delivered as an aerosol having mass medium average diameter predominantly between 1 to 5 μ, produced by a jet or ultrasonic nebulizer (or equivalent) or dry powder inhaler.

FIELD OF THE INVENTION

The current invention concerns a novel, safe, nonirritating and physiologically compatible inhalable fosfomycin plus aminoglycoside combination formulation suitable for treatment of respiratory bacterial infections caused by Gram-negative bacteria, such as Burkholderia cepacia, Citrobacter species, Escherichia coli, Enterobacter species, Fusobacterium species, Haemophilus influenzae, Klebsiella pneumoniae, Klebsiella oxytoca, Moraxella catarrhalis, Proteus inirabilis, Prevotella species, Pseudomonas aeruginosa, Serratia marcescens, Stenotrophomonas maltophilia, Alcaligenes xylosoxidans, and Gram-positive bacteria, such as methicillin-resistant Staphylococcus aureus, methicillin-sensitive Staphylococcus aureus, Streptococcus pneumoniae, and β-hemolytic Streptococcus species.

BACKGROUND OF THE INVENTION

The most widely accepted therapy for treating respiratory infections caused by Gram-negative bacteria in cystic fibrosis patients involves intravenous administration of a single antibiotic or combinations of antibiotics (Gibson et al., 2003; Ramsey, 1996). This method of treatment has several significant limitations including: (1) narrow spectrum of activity of existing antibiotics, (2) insufficient concentrations of antibiotic reaching the respiratory tract to ensure rapid onset and high rates of bacterial killing, and (3) development of adverse side affects due to high systemic concentrations of drug.

Aerosol administration of antibiotics (Conway, 2005; O'Riordan, 2000) addresses several of the limitations of parenteral administration (Flume and Klepser, 2002; Kuhn, 2001). It enables topical delivery of high concentrations of drug to the endobronchial spaces and reduces side effects by lowering systemic exposure to antibiotic. However, cystic fibrosis patients typically receive prolonged and repeated antibiotic therapies over the entire duration of their adult lives (Gibson et al., 2003; Ramsey, 1996). Therefore, cummulative aminoglycoside toxicity and development of resistance remains a significant problem.

Fosfomycin

Fosfomycin is a broad spectrum phosphonic acid antibiotic (Kahan et. al., 1974; Woodruff et al., 1977) that has bactericidal activity against Gram-negative bacteria including Citrobacter spp., E. coli, Enterobacter spp., K. pneumoniae, P. aeruginosa, Salmonella spp., Slzigella spp., and S. marcescens (Greenwood et. al., 1992; Grimm, 1979; Marchese et. al., 2003; Schulin, 2002), as well as Gram-positive bacteria including vancomycin resistant enterococci, methicillin-resistant S. aureus (MRSA), methicillin-sensitive S. aureus (MSSA), and S. pneumoniae (Greenwood et. al., 1992; Grimm, 1979; Perri et. al., 2002). Fosfomycin has the greatest activity against E. coli, Proteus spp., Salmonella spp., Shigella spp., and S. marcescens which are generally inhibited at fosfomycin concentrations ≦64 μg/mL (Forsgren and Walder, 1983). Fosfomycin is moderately active against P. aeruginosa (Forsgren and Walder, 1983), particularly when compared to tobramycin (Schulin, 2002).

Fosfomycin is bactericidal but exhibits time-dependent killing against E. coli and S. aureus (Grif et al., 2001). The rate and degree of killing depends on the length of time fosfomycin is in contact with the target organism (Craig, 1998; Mueller et al., 2004). Increasing the fosfomycin concentration will not produce a corresponding increase in the rate or degree of killing activity. This feature is significant because it is preferable to treat P. aeruginosa infections with antibiotics that exhibit bactericidal, concentration-dependent killing activity (Craig, 1998; Mueller et. al., 2004).

Fosfomycin monotherapy is commonly used in the treatment of uncomplicated urinary tract infections caused by E. coli, and less frequently in treating bacterial respiratory infections including in patients with cystic fibrosis (Kamijyo et al., 2001; Katznelson et al., 1984; Kondo et al., 1996; Reeves, 1994; Bacardi et al., 1977; Bonora et al., 1977; Honorato et al., 1977; Menendez et al., 1977). However, fosfomycin alone has not been widely used to treat infections caused by P. aeruginosa. Fosfomycin has been administered parenterally in combination with antibiotics of different classes to treat endocarditis (Moreno, et. al., 1986) and cystic fibrosis (Mirakhur et. al., 2003) but has not been delivered directly to the lung environment by aerosol administration.

Fosfomycin is available in both oral (fosfomycin calcium and fosfomycin trometamol) and intravenous (fosfomycin disodium) formulations (Woodruff et al., 1977). Fosfomycin trometamol is the preferred formulation for oral administration because it is more readily absorbed into the blood compared to fosfomycin calcium. Following a single intravenous or intramuscular dose of 2 g of fosfomycin, peak serum concentrations range between 25-95 μg/mL within 1-2 hours (Woodruff et al., 1977). By comparison, concentrations reach 1-13 μg/mL in normal lung after parenteral administration of a comparable dose of fosfomycin (Bonora et al., 1977) which is an insufficient to kill most bacterial pathogens, in particular P. aeruginosa (Forsgren and Walder, 1983; Schulin, 2002). Patients with cystic fibrosis have altered pharmacokinetics characterized by an increased volume of distribution and rate of clearance (Tan et al., 2003), which would likely further decrease the efficiacy of parenterally administered fosfomycin.

Fosfomycin is widely distributed in various body tissues and fluids but does not significantly bind to plasma proteins (Mirakhur et al., 2003). Consequently, fosfomycin is available to exert antibacterial effects if it reaches sufficient concentrations at the site of infection. The respiratory tract of cystic fibrosis patients are obstructed with viscous secretions called sputum (Ramsey, 1996). The effectiveness of several classes of antibiotics such as aminoglycosides and β-lactams is reduced due to poor penetration into sputum. Additionally, the activity of these antibiotics is further reduced by binding to sputum components (Hunt et al., 1995; Kuhn, 2001; Ramphal et al., 1988; Mendelman et al., 1985).

Development of resistance in bacteria isolated from patients treated with fosfomycin for urinary tract infections occurs very infrequently (Marchese et al., 2003). Cross-resistance with other classes of cell wall inhibiting antibiotics does not occur because fosfomycin acts on the enzyme phosphoenolpyruvate (UDP-N-acetylglucosamine enolpyruval-transferase) which is not targeted by other antibiotics (Kahan et al., 1974; Woodruff et al., 1977). Fosfomycin is actively taken up into bacterial cells by two transport systems; a constitutively functional L-α-glycerophosphate transport and the hexose-phosphate uptake system (Kahan et al., 1974). When fosfomycin resistance occurs, it is typically due to a genetic mutation in one or both of the chromosomally encoded transport systems, and less commonly by modifying enzymes (Arca et al., 1997; Nilsson et al., 2003).

Tobramycin.

Tobramycin is an aminoglycoside antibiotic that is active against Gram-negative aerobic bacilli including P. aeruginosa, E. coli Acinetobacter spp., Citrobacter spp., Enterobacter spp., K. pneumoniae, Proteus spp., Salmonella spp., S. marcescens, and Shigella spp (Vakulenko and Mobashery, 2003). In particular, tobramycin is highly active against P. aeruginosa. The tobramycin MICs of susceptible P. aeruginosa are typically less than 2 μg/mL (Shawar et al., 1999; Spencker et al., 2002; Van Eldere, 2003). Most Gram-positive bacteria are resistant to tobramycin, with the exception of S. aureus and S. epidermidis (Vakulenko and Mobashery, 2003).

Tobramycin is rapidly bactericidal and exhibits concentration-dependent killing (Vakulenko and Mobashery, 2003). Increasing the tobramycin concentration increases both the rate and extent of bacterial killing. Therefore, to achieve therapeutic success, it is necessary to administer a large enough dose to produce a peak tobramycin level 5-10 times greater than the MIC of the target organism at the site of infection. It is preferable to treat P. aeruginosa infections with antibiotics that exhibit bactericidal, concentration-dependent killing activity (Ansorg et al., 1990).

Tobramycin is usually administered to treat less serious Gram-negative bacterial infections (Vakulenko and Mobashery, 2003). However, it may be combined with other classes of antibiotics to treat severe infections of the urinary tract and abdomen, as well as endocarditis and bacteremia (Vakulenko and Mobashery, 2003). Parenteral administration of tobramycin in combination with cell-wall inhibiting antibiotics has been used to treat respiratory infections, in particular those caused by P. aeruginosa in CF patients (Gibson et al., 2003; Lang et al., 2000; Ramsey et al., 1999; Ramsey et al., 1993; Smith et al., 1999; Spencker et al., 2003).

Tobramycin is poorly absorbed orally and must be administered parenterally (Hammett-Stabler and Johns, 1998). Tobramycin is available in both intravenous and aerosol formulations. After parenteral administration, tobramycin is primarily distributed within the extracellular fluid. Tobramycin is rapidly excreted by glomular filtration resulting in a plasma half-life of 1-2 hours (Tan et al., 2003). Penetration of tobramycin into respiratory secretions is very poor and its activity is further reduced by binding to sputum (Kuhn, 2001). Aerosol administration of tobramycin results in significantly higher sputum levels of ≧1000 μg/mL (Geller et al., 2002) compared with parenteral administration, but sputum binding remains a significant problem (Hunt et al., 1995; Mendelman et al., 1985; Ramphal et al., 1988).

Nephrotoxicity and ototoxicity are adverse reactions associated with tobramycin therapy (Al-Aloui et al., 2005; Hammett-Stabler and Johns, 1998). Nephrotoxicity results from accumulation of tobramycin within lysosomes of epithelial cells lining the proximal tubules. This causes an alteration of cell function and ultimately cell necrosis (Mingeot-Leclercq and Tulkens, 1999). Clinically, this presents as nonoliguric renal failure. The prevalence of nephrotoxicity in cystic fibrosis patients is estimated to be 31-42% (Al-Aloui et al., 2005). The incidence of ototoxicity, which is characterized by loss of hearing and dizziness, is estimated to be as high as 25% of patients treated with aminoglycosides (Hammett-Stabler and Johns, 1998). Unlike nephrotoxicity, ototoxicity is irreversible. The greatest risk factor for the development of toxicity is cumulative exposure to large doses of tobramycin (Hammett-Stabler and Johns, 1998; Mingeot-Leclercq and Tulkens, 1999). Cystic fibrosis patients are treated with prolonged and repeated high-dosages of tobramycin over their entire lifetime (Tan et al., 2003) and are at increased risk of developing cumulative renal failure (Al-Aloui et al., 2005).

Bacterial resistance to tobramycin has become increasingly prevalent and is due to repeated and prolonged antibiotic monotherapy (Conway et al., 2003; Van Eldere, 2003; Mirakhur et al., 2003; Pitt et al., 2005; Schulin, 2002). For example, Cystic fibrosis patients are colonized with P. aeruginosa strains which are largely resistant to tobramycin gentamicin, ceftazidinme, piperacillin, and ciprofloxacin (Eldere, 2003; Pitt et al., 2005; Pitt et al., 2003; Weiss and Lapointe, 1995). Thus, existing antibiotic therapies are becoming ineffective for treating P. aeruginosa infections because of drug resistance.

It is clear that there is a continued need for an improved method of treatment for acute and chronic respiratory infections caused by Gram-negative and Gram-positive bacteria, particularity multidrug resistant P. aeruginosa. This is particularly evident in cystic fibrosis patients where current therapies are limited by problems with development of resistance and toxicity. Such method of treatment would preferably comprise inhalation of an aerosolized antibiotic combination of fosfomycin and an aminoglycoside such as tobramycin that delivers a therapeutically effective amount of the drugs directly to the endobronchial space of the airways or to the nasal passages. Such treatment would be efficacious, reduce the frequency of drug resistance, and improve safety.

It would be highly advantageous to provide a formulation and system for delivery of a sufficient dose of fosfomycin plus an aminoglycoside such as tobramycin in a concentrated form, containing the smallest possible volume of solution or weight of dry powder which can be aerosolized and delivered predominantly to the endobronchial space.

Thus, it is an objective of this invention to provide a concentrated liquid or dry powder formulation of fosfomycin plus aminoglycoside which contains sufficient but not excessive amounts of fosfomycin and aminoglycoside which can be efficiently aerosolized by nebulization into aerosol particles sizes predominantly within a range of 1 to 5 um and having salinity that is adjusted to permit generation of a fosfomycin plus aminoglycoside aerosol well tolerated by patients, and which has an adequate shelf live.

SUMMARY OF THE INVENTION

One aspect of this invention is a method for treatment of upper respiratory tract infections like bacterial sinusitis and lower respiratory tract (pulmonary) infections like cystic fibrosis, chronic pulmonary Pseudomonas infection in cystic fibrosis patients, Pseudomonas infections after first infection, bronchiectasis, hospital and community acquired pneumonia, and ventilator associated pneumonia (VAP) caused by Gram-negative and Gram-positive bacteria comprising the step of administering an effective amount of an aerosol formulation comprising fosfomycin and tobramycin into the endobronchial or nasal space of patients in need of such treatment.

Another aspect of the current invention is a concentrated formulation either liquid or dry powder, suitable for delivery of a fosfomycin tobramycin combination into the endobronchial or nasal space of patients to treat lower and upper respiratory bacterial infections.

Still another aspect of the current invention is a formulation, either liquid or dry powder, suitable for delivery of aminoglycoside and fosfomycin into the endobronchial and nasal space of patients that reduces the development of antibiotic resistance compared to either drug used alone.

Still another aspect of the current invention is a formulation, either liquid or dry powder, suitable for delivery of an aminoglycoside and fosfomycin into the endobronchial or nasal space of patients that increases the post antibiotic affect (PAE) compared to the PAE of either the drug used alone.

Still another aspect of the current invention is a formulation comprising from 1-300 mg of fosfomycin and from 1 to 300 mg aminoglycoside in 0.5 to 7 mL of water with a chloride concentration >30 mM wherein said formulation has a pH between 4.5 and 8.0 and is delivered by aerosolization. The aerosol contains particles that have a mass medium average diameter (MMAD) predominantly between 1 to 5 g and is administered using a nebulizer able to atomize the particles of the required size.

Still another aspect of the current invention is a dry powder formulation comprising from about 1 to about 300 mg of fosfomycin, from about 1 to about 300 mg aminoglycoside and at least one pharmaceutically acceptable excipient in a micronized dry powder form that delivers particles with a mass medium average diameter MMAD of 1 and 5μ upon aerosolization.

The invention concerns an inhalable formulation comprising fosfomycin plus an aminoglycoside suitable for treatment of acute and chronic pulmonary bacterial infections, particularly those caused by the multidrug resistant Gram-negative bacteria P. aeruginosa which are resistant to treatment with β-lactams and aminoglycosides. The aerosol formulation has a small volume yet delivers a therapeutically efficacious dose of fosfomycin plus aminoglycoside to the site of infection in amounts sufficient to treat bacterial respiratory infections. The dry powder and reconstituted aminoglycoside plus fosfomycin formulations have a long shelf-life and storage stability.

The formulations of the present invention preferably comprise from 5 to 9 parts by weight fosfomycin and 1 to 9 parts by weight aminoglycoside, preferably from about 7 to 9 parts by weight fosfomycin and 1 to 3 parts by weight aminoglycoside and more preferably about 8 parts by weight fosfomycin and about 2 parts by weight aminoglycoside. The most preferred aminoglycoside is tobramycin.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 and 2 show time-kill curves for a 9:1 fosfomycin:tobramycin combination and demonstrate rapid bactericidal killing of P. aeruginosa ATCC 27853 compared to bacteriostatic killing of fosfomycin and tobramyicn alone.

FIGS. 3 and 4 show time-kill curves for 8:2 fosfomycin:tobramycin combinations and demonstrate rapid bactericidal killing of P. aeruginosa ATCC 27853.

FIGS. 5, 6 and 7 show time-kill curves for 7:3 fosfomycin:tobramycin combinations and demonstrate rapid bactericidal killing of P. aeruginosa ATCC 27853.

FIGS. 8 and 9 compare kill curves for 9:1, 8:2, and 7:3 fosfomycin:tobramycin combinations and tobramycin alone against P. aeruginosa ATCC 27853.

FIGS. 10, 11 and 12 demonstrate concentration-dependent killing of 9:1, 8:2, and 7:3 fosfomycin:tobramycin combinations against P. aeruginosa ATCC 27853.

FIGS. 13, 14, and 15 show the killing of E. coli in the rat lung after aerosol administration of a 31.1 mg/mL solution of a 9:1, 8:2, and 7:3 fosfomycin:tobramycin combinations, respectively.

FIGS. 16 and 17 demonstrate that aerosol administration of a 31.1 mg/mL solution of a 9:1 fosfomycin:tobramycin combination resulted in >5 log10 killing of E. coli in the rat lung, compared to less than 1 log10 killing fosfomycin and tobramycin alone.

FIG. 18 demonstrates that aerosol administration of a 31.1 mg/mL solution of a 8:2 fosfomycin:tobramycin combination resulted in >5 log10 killing of E. coli in the rat lung, compared to 2 log10 killing by aerosol administration of a 40 mg/mL solution of tobramycin alone.

FIGS. 19 and 20 demonstrate that aerosol administration of a 31.1 mg/mL solution of a 8:2 fosfomycin:tobramycin combination resulted in greater killing of P. aeruginosa in the rat lung, compared to fosfomycin and tobramycin alone.

FIG. 21 demonstrates that aerosol administration of a 60 and 90 mg/mL solution of a 8:2 fosfomycin:tobramycin combination resulted in significant killing of P. aeruginosa in the rat lung.

DEFINITION OF TERMS

As used herein:

“Quarter normal saline” or “¼ NS” means normal saline diluted to its quarter strength containing 0.225% (w/v) NaCl.

“9:1 fosfomycin:tobramycin” means a water solution or dry powder formulation containing a 9:1 ratio by weight of fosfomycin acid to tobramycin base.

“8:2 fosfomycin:tobramycin” means a water solution or dry powder formulation containing a 8:2 ratio by weight of fosfomycin acid to tobramycin base such that the amount of fosfomycin is eight times the amount of tobramycin.

“7:3 fosfomycin:tobramycin” means a water solution or dry powder formulation containing a 7:3 ratio by weight of fosfomycin acid to tobramycin base.

“5:5 fosfomycin:tobramycin” means a water solution or dry powder formulation containing a 5:5 ratio by weight of fosfomycin acid to tobramycin base.

“Minimal inhibitory concentration (MIC)” means the lowest concentration of antibiotic (s) that prevents visible growth after incubation for 18-20 hours at 35° C.

“Minimal bactericidal concentration (MBC)” means the lowest concentration of antibiotic that results in >3 Log10 of bacterial killing.

“Time-dependent killing” means higher drug concentrations do not kill bacteria any faster or to a greater extent.

Concentration-dependent killing” means the higher the drug concentration, the greater the rate and extent of bacterial killing.

“Bacteriostatic” means the antibiotic acts by inhibiting bacterial growth.

“Bactericidal” means the antibiotics acts by killing bacteria.

“Synergy” means the combined effect of the antibiotics being examined is significantly greater than either drug alone.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The current invention concerns a concentrated fosofomycin plus tobramycin formulation suitable for efficacious delivery by aerosolization into the endobronchial or nasal space. The invention is preferably suitable for formulation of concentrated fosfomycin plus tobramycin for aerosolization by jet, ultrasonic, or equivalent nebulizers to produce aerosol particles between 1 and 5μ necessary for the efficacious delivery of fosfomycin plus tobramycin into the endobronchial or nasal space to treat bacterial infections, particularly those caused by multidrug-resistant Pseudomonas aeruginosa. The formulation contains a minimal yet efficacious amount of fosfomycin plus tobramycin formulated in the smallest possible volume of physiologically acceptable solution having a salinity adjusted to permit generation of a fosfomycin and tobramycin aerosol well tolerated by patients but minimizing the development of secondary undesirable side effects such as bronchospasm and cough.

Primary requirements for any aerosolized formulation are its safety and efficacy. Additional advantages are lower cost, practicality of use, long shelf-life, storage and manipulation of nebulizer.

Aerosolized fosfomycin plus tobramycin is formulated for efficacious delivery of fosfomycin plus tobramycin to the lung endobronchial or nasal space. A nebulizer is selected to allow the formation of a fosfomycin plus tobramycin aerosol having a mass medium average diameter predominantly between 1 to 5μ. The formulated and delivered amount of fosfomycin plus tobramycin is efficacious for the treatment of bacterial pulmonary infections, particularly those caused by multi-drug resistant Pseudomonas aeruginosa. The formulation has salinity, osmotic strength, and pH adjusted to permit generation of a fosfomycin plus tobramycin aerosol that is well tolerated by patients. The formulation has the smallest possible aerosolizable volume able to deliver an effective dose of fosfomycin plus tobramycin to the site of the infection. Additionally, the aerosolized formulation does not impair the functionality of the airways or nasal passages and minimizes undesirable side effects.

I. Evaluation of the Antibiotic Combinations

Fosfomycin:aminoglycosides combinations were formulated as described in Example 1. Fosfomycin comprised the major component of the combination because of it's inherent safety and the need to reduce the toxicity of the aminoglycoside. The fosfomycin:aminoglycoside combinations, in particular fosfomycin:tobramycin were evaluated for (a.) in vitro potency, (b.) killing rates, (c.) frequency of resistance, and (d.) animal efficacy.

a. In Vitro Potency

The in vitro potency of fosfomycin and tobramycin alone and in combination against a panel of Gram-negative and Gram-positive bacteria representative of species that cause respiratory tract infections in cystic fibrosis, bronchiectasis, sinusitis and ventilator-associated pneumoniae is shown in Tables 1 and 2. The data show that the fosfomycin:tobramycin combinations have antibacterial activity against a broad spectrum of Gram-positive and Gram-negative bacteria. The MICs of the fosfomycin:tobramycin combinations did not improve over fosfomycin or tobramycin alone. TABLE 1 MIC values of fosfomycin and tobramycin alone and in combination against Gram-negative bacteria. MIC (μg/mL) Bacteria Fosfomycin Tobramycin F9:T1 F7:T3 F5:T5 P. aeruginosa COR-003 16 0.5 8 4 1 COR-009 1024 2048 1024 512 2048 COR-021 8 64 8 8 8 COR-027 128 0.5 8 2 1 E. coli COR-032 1 0.5 1 1 1 COR-039 64 0.25 2 0.5 0.5 COR-040 1 32 1 1 2 H. influenzae COR-042 8 0.5 4 2 1 COR-049 128 0.5 4 1 1 B. cepacia COR-090 ≧2048 128 1024 ND ND COR-098 ≧2048 64 1024 ND ND S. maltophilia COR-082 128 4 4 4 8 COR-083 64 512 256 128 128 COR-087 ≧512 128 128 128 128 K. pneumoniae COR-042 8 0.5 4 2 1 COR-049 128 0.5 4 1 1 M. catarrhalis COR-109 4 8 4 ND ND COR-113 4 8 4 ND ND ND = Not determined.

TABLE 2 MIC values of fosfomycin and tobramycin alone and in combination against Gram-positive bacteria. MIC (μg/mL) Bacteria Fosfomycin Tobramycin F9:T1 F7:T3 F5:T5 S. aureus COR-051 1 0.25 2 1 0.5 (MSSA) COR-055 4 0.25 4 1 0.5 (MRSA) COR-059 2 128 2 4 4 COR-060 2 64 1 4 8 (GISA) S. pneumoniae COR-061 32 64 16 32 32 COR-068 8 16 16 16 16 S. pyogenes COR-104 64 32 ND ND ND COR-105 16 64 ND ND ND E. faecalis COR-099 32 512 ND ND ND COR-103 32 8 ND ND ND ND = Not determined.

Table 3 shows the MIC₅₀ and MIC₉₀ values of fosfomycin and tobramycin alone and in combination for 100 P. aeruginosa strains isolated from lung sputum samples from cystic fibrosis patients. This study demonstrated that in the absence of mucin, tobramycin was the most active antibiotic. Combining fosfomycin and tobramycin resulted in a significant improvement in the MIC₅₀ and MIC₉₀ values compared to fosfomycin alone. In the CF mucin binding model (+mucin), the fosfomycin:tobramycin combinations had MIC₅₀ and MIC₉₀ values to comparable tobramycin alone. TABLE 3 MIC values of fosfomycin and tobramycin alone and in combination for 100 P. aeruginosa strains. MIC₅₀ (μg/mL) MIC₉₀ (μg/mL) Antibiotic Mucin (−) Mucin (+) Mucin (−) Mucin (+) Fosfomycin 64 64 ≧512 128 Tobramycin 2 16 16 64 F9:T1 16 32 64 128 F8:T2 8 32 64 128 F7:T3 8 16 64 64

Table 4 shows the results of checkerboard synergy studies between fosfomycin and nine aminoglycosides. The interactions between the nine different fosfomycin:aminoglycoside combinations were all classified as indifferent. None of the combinations had synergistic activity against P. aeruginosa ATCC 27853. This data demonstrates it is not obvious that an improvement in efficacy would be achieved by combining fosfomycin and aminoglycosides. TABLE 4 Checkerboard studies between fosfomycin and various aminoglycosides for P. aeruginosa ATCC 27853. Antibiotic Combination Interaction Fosfomycin:Tobramycin Indifferent Fosfomycin:Gentamicin Indifferent Fosfomycin:Arbekacin Indifferent Fosfomycin:Dibekacin Indifferent Fosfomycin:Kanamycin B Indifferent Fosfomycin:Streptomycin Indifferent Fosfomycin:Amikacin Indifferent Fosfomycin:Neomycin Indifferent Fosfomycin:Netilmicin Indifferent

Table 5 shows the results of checkerboard synergy studies between fosfomycin and tobramycin against clinical strains of P. aeruginosa, E. coli, H. influenzae, and S. aureus. Interactions between fosfomycin and tobramycin were classified as indifferent for the vast majority of isolates examined. The interaction between fosfomycin and tobramycin combination was synergistic against only one strain of P. aeruginosa and one strain of E. coli. This data further suggests it was not obvious that an improvement in efficacy would be achieved by combining fosfomycin and tobramycin. TABLE 5 Checkerboard studies between fosfomycin and tobramycin for P. aeruginosa, E. coli, H. influenzae, and S. aureus strains. Bacterial Antibiotic Interaction Strains N= Synergism Addition Indifference Antagonism P. aeruginosa 17 1 0 16 0 E. coli 5 1 0 4 0 H. influenzae 1 0 0 1 0 S. aureus 4 0 0 4 0

Table 6 shows the MBC/MIC values of fosfomycin and tobramycin alone and 9:1, 8:2, and 7:3 fosfomycin:tobramycin combinations for P. aeruginosa ATCC 27853. Antibiotics with MBC/MIC values ≦1 are preferable because they inhibit bacteria by killing rather slowing growth. This study unexpectedly demonstrated the MBC/MIC values of the 9:1, 8:2, and 7:3 fosfomycin:tobramycin combinations were identical to tobramycin alone. This finding was unexpected since the fosfomycin alone MBC/MIC ratio was ≧8. The study also showed that a similar finding was not observed with any of the other fosfomycin:aminoglycoside combinations. TABLE 6 MBC/MIC values of aminoglycosides alone and 9:1, 8:2, and 7:3 combinations of fosfomycin and aminoglycoside for P. aeruginosa ATCC 27853. MBC/MIC Antibiotic Amino- Combination glycoside F9:T1 F8:T2 F7:T3 Fosfomycin:Tobramycin 1 1 1 1 Fosfomycin:Gentamicin 2 4 2 8 Fosfomycin:Amikacin 2 4 4 2 Fosfomycin:Netilimicin 1 2 4 2 Fosfomycin:Arbekacin 1 4 4 2 Fosfomycin:Streptomycin 4 2 2 2 Fosfomycin:Neomycin 2 2 2 4 Fosfomycin:Kanamycin B ND 4 2 4 Fosfomycin:Dibekacin 1 4 2 1 ND = Not determined.

Table 7 shows the MBC/MIC values of fosfomycin and tobramycin alone and 9:1, 8:2, and 7:3 combinations for P. aeruginosa ATCC 27853, E. coli ATCC 25922, and S. aureus ATCC 29213. For P. aeruginosa, the MBC/MIC values of the 9:1, 8:2, and 7:3 combinations were identical to tobramycin alone. This finding was unexpected since it was not observed with E. coli or S. aureus. TABLE 7 MBC/MIC values of fosfomycin and tobramycin alone and 9:1, 8:2, and 7:3 combinations of fosfomycin and tobramycin. MBC/MIC Antibiotic P. aeruginosa E. coli S. aureus Fosfomycin ≧8 8 1 Tobramycin 1 1 8 F9:T1 1 1 2 F8:T2 1 2 4 F7:T3 1 2 4 b. Killing Rates

Table 8 shows the results of time-kill studies of 9:1, 8:2, and 7:3 fosfomycin:aminoglycoside combinations against P. aeruginosa ATCC 27853. Examination of bacterial killing over time revealed synergy between fosfomycin:tobramycin, fosfomycin:gentamicin, and fosfomycin:arbekacin combinations. The remaining fosfomycin:aminoglycoside combinations were not synergistic. This data shows that it was not obvious which combination of fosfomycin:aminoglycoside, nor which ratio of fosfomycin:aminoglycoside would be synergistic. TABLE 8 Time-kill studies with 9:1, 8:2, and 7:3 fosfomycin:aminoglycoside combinations against P. aeruginosa ATCC 27853. Antibiotics were evaluated at a concentration of 32 μg/mL. Antibiotic Synergy Combination F9:T1 F8:T2 F7:T3 Fosfomycin:Tobramycin Yes No No Fosfomycin:Gentamicin Yes No No Fosfomycin:Amikacin No No No Fosfomycin:Netilimicin No No No Fosfomycin:Arbekacin No No Yes Fosfomycin:Streptomycin No No No Fosfomycin:Neomycin No No No Fosfomycin:Kanamycin B No No No Fosfomycin:Dibekacin No No No

Table 9 shows the time to achieve bactericidal killing of P. aeruginosa ATCC 27853 by various fosfomycin:aminoglycoside combinations. Antibiotics were evaluated at a concentration of 32 μg/mL. This study demonstrated that the 9:1, 8:2, and 7:3 fosfomycin:tobramycin combinations achieved bactericidal killing faster than any other fosfomycin:aminoglycoside combination. The data also showed that three of the fosfomycin:aminoglycoside combinations did not exhibit bactericidal killing. These results were unexpected because tobramycin alone reached bactericidal killing at 1 hour, and fosfomycin alone was not bactericidal. TABLE 9 Time to reach bactericidal killing of P. aeruginosa ATCC 27853 by 9:1, 8:2, and 7:3 fosfomycin:aminoglycoside combinations. Antibiotic Time (h) Combination F9:T1 F8:T2 F7:T3 Fosfomycin:Tobramycin 2 1 1 Fosfomycin:Gentamicin 6 4 2 Fosfomycin:Amikacin 6 2 2 Fosfomycin:Netilimicin NC NC NC Fosfomycin:Arbekacin NC 4 2 Fosfomycin:Streptomycin NC NC NC Fosfomycin:Neomycin NC NC NC Fosfomycin:Dibekacin NC 4 4

FIGS. 1 and 2 show time-kill curves for a 9:1 fosfomycin:tobramycin combination and demonstrate rapid bactericidal killing of P. aeruginosa ATCC 27853 compared to bacteriostatic killing of fosfomycin and tobramyicn alone. This was unexpected since checkerboard analysis did not demonstrate synergy between fosfomycin and tobramycin.

FIGS. 3 and 4 show time-kill curves for 8:2 fosfomycin:tobramycin combinations and demonstrate rapid bactericidal killing of P. aeruginosa ATCC 27853. At a concentration of 32 μg/mL, the killing activity of the combination was significantly superior to fosfomycin and slightly superior to tobramycin. At 16 μg/mL, the combination was superior to tobramycin and fosfomycin alone.

FIGS. 5, 6 and 7 show time-kill curves for 7:3 fosfomycin:tobramycin combinations and demonstrate rapid bactericidal killing of P. aeruginosa ATCC 27853. At concentrations of 16 μg/mL and 32 μg/mL, the killing activity of the combination was significantly superior to fosfomycin and slightly superior than tobramycin alone. At 8 μg/mL, the combination was superior to both tobramycin and fosfomycin alone.

FIGS. 8 and 9 compare kill curves for 9:1, 8:2, and 7:3 fosfomycin:tobramycin combinations and tobramycin alone against P. aeruginosa ATCC 27853. At a concentration of 32 ug/mL, all the antibiotics had comparable rates and degrees of killing. At 16 ug/mL, tobramycin was the most active antibiotic followed by F7:T3, F8:T2, and F9:T1.

FIGS. 10, 11 and 12 demonstrate concentration-dependent killing of 9:1, 8:2, and 7:3 fosfomycin:tobramycin combinations against P. aeruginosa ATCC 27853. This was unexpected because the major component of the combination, fosfomycin, exhibits time-dependent killing. Tobramycin alone exhibits concentration-dependent killing.

c. Frequency of Resistance

Table 10 shows the frequency of development of resistance to fosfomycin and tobramycin alone, and a 9:1 fosfomycin:tobramycin combination for five P. aeruginosa strains. This study demonstrated the frequency of development of resistance to the 9:1 fosfomycin:tobramycin combination was 1000-100,000 times less than fosfomycin alone and 10-1000 times less than tobramycin alone. TABLE 10 Frequency of development of resistance to fosfomycin, tobramycin and a 9:1 fosfomycin:tobramycin combination. P. aeruginosa Frequency of Resistance Strains Fosfomycin Tobramycin Fos + Tob COR-002 1.4 × 10⁻³ 6.7 × 10⁻⁶ <2.9 × 10⁻⁸ COR-003 5.5 × 10⁻⁴ 3.2 × 10⁻⁶ <1.2 × 10⁻⁹ COR-013 6.4 × 10⁻³ 2.6 × 10⁻⁶  2.0 × 10⁻⁷ COR-014 1.5 × 10⁻⁵ 5.0 × 10⁻⁶ <1.4 × 10⁻⁹ ATCC 27853 4.1 × 10⁻⁵ 3.4 × 10⁻⁶ <2.5 × 10⁻⁹

Table 11 shows the fold increase in the MIC of P. aeruginosa mutants isolated after a single exposure to fosfomycin, tobramycin, or a 9:1 combination of fosfomycin:tobramycin. This study demonstrated that the 9:1 mutants did not have a significant increase in MIC (1-2 fold), compared to fosfomycin (128->512 fold) or tobramycin (2-16 fold) mutants. TABLE 11 Fold increase in MIC after a single exposure to antibiotic. P. aeruginosa Fold Increase in MIC Strain F9:T1 Fos Tob COR-002 1 >256 1 COR-003 2 >256 2 COR-013 1 >64 8 COR-014 1 >512 16 ATCC 27853 2 128 1

Table 12 shows the development of resistance in a clinical P. aeruginosa strain after 28 days of continuous exposure to fosfomycin, tobramycin or a 9:1 fosfomycin:tobramycin combination. Exposure to the 9:1 combination caused a 8-fold increase in MIC by day 14, but the MIC did not exceed the fosfomycin resistance breakpoint of 256 μg/mL. Exposure to tobramycin increased the MIC 128-fold which exceeded the tobramycin resistance breakpoint of 16 μg/mL. A rapid and dramatic development of resistance was observed with P. aeruginosa exposed to fosfomycin alone. TABLE 12 Development of resistance after continuous exposure to antibiotic. MIC (μg/mL) Antibiotic 0 d 7 d 14 d 21 d 28 d F9:T1 8 16 64 64 64 Tobramycin 0.5 0.5 0.5 32 64 Fosfomycin 16 >512 >512 >512 >512 d. Animal Efficacy

FIGS. 13, 14, and 15 show the killing of E. coli in the rat lung after aerosol administration of a 31.1 mg/mL solution of a 9:1, 8:2, and 7:3 fosfomycin:tobramycin combinations, respectively. This study demonstrated complete eradication (5-6 Log10 CFU) of the E. coli lung infection after 45-60 minutes of treatment.

FIGS. 16 and 17 demonstrate that aerosol administration of a 31.1 mg/mL solution of a 9:1 fosfomycin:tobramycin combination resulted in >5 log10 killing of E. coli in the rat lung, compared to less than 1 log10 killing fosfomycin and tobramycin alone.

FIG. 20 demonstrates that aerosol administration of a 31.1 mg/mL solution of a 8:2 fosfomycin:tobramycin combination resulted in >5 log10 killing of E. coli in the rat lung, compared to 2 log10 killing by tobramycin alone. This was unexpected because the in vitro potency data did not demonstrate that the combination was superior to tobramycin alone.

FIGS. 19 and 20 demonstrate that aerosol administration of a 31.1 mg/mL solution of a 8:2 fosfomycin:tobramycin combination resulted in greater killing of P. aeruginosa in the rat lung, compared to fosfomycin and tobramycin alone.

FIG. 21 demonstrates that aerosol administration of a 60 and 90 mg/mL solution of a 8:2 fosfomycin:tobramycin combination resulted in significant killing of P. aeruginosa in the rat lung.

II. Aerosol Fosfomycin/Tobramycin Formulation

Aminoglycosides useful in the invention are antibiotics, such as tobramycin, gentamicin, kanamycin B, amikacin, arbekacin, dibekacin, streptomycin, neomycin, and netilmicin. Fosfomycin compounds particularly useful in the invention are antibiotics, such as fosfomycin trometamol, fosfomycin disodium salt, and fosfomycin calcium.

The preferred fosfomycin plus tobramycin formulation according to the invention contains 10-500 mg of fosfomycin plus tobramycin per 0.5-7 mL of water with a chloride concentration >30 mM. This corresponds to doses that would be required to prevent colonization or to treat severe upper and lower respiratory tract infections caused by a range of susceptible organisms.

Patients can be sensitive to the pH, osmolarity, and ionic content of a nebulized solution. Therefore these parameters should be adjusted to be compatible with fosfomycin plus tobramycin and tolerable to patients. The most preferred solution or suspension of fosfomycin plus tobramycin will contain a chloride concentration >30 mM at pH 4.5-8.0.

The formulation of the invention is nebulized predominantly into particle sizes allowing delivery of the drug into the terminal and respiratory bronchioles or nasal passages where the bacteria reside during infection and colonization. For efficacious delivery of fosfomycin plus tobramycin to the lung endobronchial space of airways in an aerosol, the formation of an aerosol having a mass medium average diameter predominantly between 1 to 5μ is necessary. The formulated and delivered amount of fosfomycin plus tobramycin for the treatment and prophylaxis of endobronchial infections, particularly those caused by the P. aeruginosa must effectively target the lung surface. The formulation must have the smallest possible aerosolizable volume able to deliver an effective dose of fosfomycin plus tobramycin to the site of the infection. The formulation must additionally provide conditions which would not adversely affect the functionality of the airways and nasal passages. Consequently, the formulation must contain enough of the drug formulated under the conditions to allow its efficacious delivery, while avoiding undesirable reactions. The new formulation according to the invention meets all these requirements.

According to the invention, fosfomycin plus tobramycin is formulated in a liquid dosage form intended for inhalation therapy by patients with, or at risk to acquire, bacterial upper or lower respiratory infection. Since the patients reside throughout the world, it is imperative that the liquid dosage formulation has reasonably long shelf-life. Storage conditions and packaging thus become important.

The formulation of fosfomycin plus tobramycin can be aseptically prepared as an aqueous solution in a dual blow-fill ampoule such that each antibiotic is independently formulated in water and pH adjusted. Tobramycin is formulated at acidic pH (1-6) while fosfomycin is formulated at basic pH (8-13). The low pH formulation for tobramycin assures that all of its basic nitrogen atoms are protonated thus protecting the molecule from amine oxidation and degradation (tobramycin solutions turn yellow at room temperature). Thus, the low pH solutions of tobramycin are stable at room temperature for indefinite periods of time. Fosfomycin is most stable at high pH as the reactive epoxide ring is prone to hydrolysis (ring opening) at acidic or low pH. High pH solutions of fosfomycin are also stable at room temperature. Therefore, the dual ampoule blow-fill container allows for the separate and room temperature stable formulation of each antibiotic at high and low pH such that the drug product combination and final pH is created by mixing both solutions in the nebulizer immediately before use. The storage suitability of the formulation allows reliable usage of the formulated fosfomycin plus tobramycin suitable for aerosolization.

III. Nebulizers for Pulmonary Delivery of the Antibiotic Combination

A device able to nebulize the formulation of the invention into aerosol particles predominantly in the 1 to 5μ size range is utilized to administer the formulations of the present invention. Predominantly in this application means that at least 70% but preferably more than 90% of all generated aerosol particles are within the 1 to 5μ range. Typical devices include jet nebulizers, ultrasonic nebulizers, pressurized aerosol generating nebulizers, and vibrating porous plate nebulizers. Representative suitable nebulizers include the eFlow® nebulizer available from Pari Inovative Manufactures, Midlothian, Va.; the iNeb® nebulizer available from Profile Drug Delivery of West Sussex, United Kingdom; the Omeron MicroAir® nebulizer available from Omeron, Inc. of Chicago, Ill. and the AeroNebGo® nebulizer available from Aerogen Inc. of Mountain View, Calif.

A jet nebulizer utilizes air pressure to break a liquid solution into aerosol droplets. An ultrasonic nebulizer works by a piezoelectric crystal that shears a liquid into small aerosol droplets. Pressurized systems general force solutions through small pores to generate small particles. A vibrating porous plate device utilizes rapid vibration to shear a stream of liquid into appropriate droplet sizes. However, only some formulations of fosfomycin plus tobramycin can be efficiently nebulized as the devices are sensitive to the physical and chemical properties of the formulation.

The invention is a small volume, high concentration formulation of fosfomycin plus tobramycin that can be delivered as an aerosol at efficacious concentrations of the drug to the respiratory tract in people at risk for, or suffering from infection caused by susceptible bacteria. The formulation is safe, well tolerated, and very cost effective. Furthermore, the formulation provides adequate shelf life for commercial distribution. The foregoing may be better understood from the following examples, which are presented for the purposes of illustration and are not intended to limit the scope of the inventive concepts.

EXAMPLE 1 Preparation of Fosfomycin/Tobramycin Solutions for Aerosolization

9:1 Fosfomycin/Tobramycin Solution Fosfomycin disodium (18.057 g, 13.99 g free acid) was dissolved in 250 mL of water and the pH was adjusted to 7.41 by the dropwise addition of 1.53 mL of 4.5 N HCl. To the resulting solution was added 1.56 g of 97.5% tobramycin base. The pH of the solution was adjusted to 7.60 by the addition of 2.45 mL of 4.5 N HCl. The solution was diluted to 500 mL with water and filtered through a 0.2 μm Nalge Nunc 167-0020 membrane filter. The final pH was 7.76, the osmolality was 537 mOsmol/kg, the fosfomycin/tobramycin ratio was calculated to be 9:1, and the chloride concentration was 35.8 mM.

8:2 Fosfomycin/Tobramycin Solution A solution of fosfomycin/tobramycin in a 8:2 ratio was prepared. 3.1680 g of fosfomycin disodium (2.4013 g free acid) was dissolved in 50 ml water. 0.6154 g of 97.5% tobramycin base (0.6000 g of pure tobramycin base) was dissolved in the fosfomycin solution. The pH adjusted by adding 0.910 mL of 6 M HCl. The solution was diluted to 100 mL with water. The final pH of the solution was 7.65, osmolality was 477 mOsmol/kg, and the chloride concentration was 54.6 mM. The final fosfomycin/tobramycin ratio was calculated to be 8:2

7:3 Fosfomycin/Tobramycin Solution Using the procedure described for the 9:1 solution, above, a solution of fosfomycin/tobramycin in a 7:3 ratio was prepared; 17.466 g of fosfomycin disodium (13.239 g free acid) was dissolved in water; the pH adjusted to 7.43 by adding 1.46 mL of 4.5 N HCl, 5.819 g of 97.5% tobramycin base (5.674 g of pure tobramycin base), and the pH of the combined solution was adjusted by adding 9.20 mL of 4.5 N HCl. The final pH of the solution was 7.68, the osmolality was 560 mOsmol/kg, the fosfomycin/tobramycin ratio was 7:3, and the chloride concentration was 95.9 mM.

EXAMPLE 2 Preparation of A 8:2 Hyperosmolar Solution of Fosfomycin and Tobramycin

7.9165 g of fosfomycin disodium (6.0007 g of fosfomycin free acid) and 1.5382 g of 97.5% tobramycin base (1.4997 g of pure tobramycin base) were dissolved in 50 mL water. The pH was adjusted to 7.62 by the addition of 2.3 mL of 6 M HCl. The combined solution was diluted to 100 mL. The final pH was 7.64, the osmolality was 1215 mOsmol/kg, the final chloride concentration was 138 mM, and the fosfomycin/tobramycin ratio was 8:2.

EXAMPLE 3 Preparation of Individual Solutions of High pH Fosfomycin and Low pH Tobramycin for Reconstitution

A fosfomycin solution was prepared by dissolving 5.891 g of fosfomycin disodium (4.465 g of fosfomycin free acid) in water and diluting to 100 mL. The pH was 9.42, and the osmolality was 795 mOsmol/kg.

A tobramycin solution was prepared by dissolving 1.869 g of 97.5% tobramycin base (1.822 g of pure tobramycin base) in 60 mL water, adjusting the pH to 4.90 by adding 18.8 mL of 1 M HCl, and diluting to 100 ml with water. The pH was 4.89, and the osmolality was 148 mOsmol/kg.

1 mL of the fosfomycin solution and 1 mL of the tobramycin solution were combined. For the combined drug product solution, the pH was 7.30, the osmolality was 477 mOsmol/kg, the chloride concentration was 94.0 mM, and the fosfomycin/tobramycin ratio was 7:3.

EXAMPLE 4 Preparation of an 8:2 Fosfomycin/Tobramycin Solution from Commercial Tobramycin for Inhalation Solution (TOBI) and Lyophilized (Dry) Fosfomycin Disodium (FOSFO)

3.1665 g of fosfomycin disodium (FOSFO, 2.4002 g of fosfomycin free acid) was dissolved in two 5 mL (60 mg/mL) ampoules of Tobramycin Solution for Inhalation (TOBI, 600 mg of tobramycin base) plus 40 mL water. The initial pH of the solution was 7.50. To the solution was added 667 μl of 4.5 M NaCl. The combined solution was diluted to 100 mL with water. The final formulation had the following properties; pH 7.49, osmolality 502 mOsmol/kg, chloride concentration 33.8 mM, and the fosfomycin/tobramycin ratio was 8:2, and drug concentration of 24 mg/mL fosfomycin and 6 mg/mL of trobramycin.

Examples 5-10 set forth the procedures that have been used to generate the date presented in the Tables and Figures of this application.

EXAMPLE 5 Determination of Minimal Inhibitory Concentrations (MIC)

The efficacy of antibiotics and antibiotic combinations against Gram-positive and Gram-negative bacteria representative of species that cause respiratory infections in patients with cystic fibrosis, bronchiectasis, sinusitis, and ventilator-associated pneumonia were evaluated in MIC assays. P. aeruginosa strains were isolated from lung sputum samples collected from cystic fibrosis patients, blood cultures, respiratory tract infections, and skin or soft tissue infections. E. coli, H. influenzae, B. cepacia, S. maltophilia, K. pneumoniae, M. catarrhalis, S. aureus, S. pneumoniae, S. pyogenes, and E. faecalis were isolated from respiratory tract infections. E. coli ATCC 25922, P. aeruginosa ATCC 27853, and S. aureus ATCC 29213 were used as quality control stains.

Method A: The MICs of fosfomycin alone, tobramycin alone, or combinations of fosfomycin plus tobramycin were determined by the agar-plate dilution method according to NCCLS guidelines (NCCLS, 2003). Bacterial strains were streaked onto Trypic Soy Agar plates (PML Microbiologicals, Wilsonville, Or.) containing 5% defibrinated sheep blood (hereafter referred to as blood agar plates) and incubated overnight at 35° C. Two to three bacterial colonies from the overnight cultures were inoculated into 3 mL of sterile normal saline, vortexed briefly, and adjusted to a 0.5 McFarland standard (NCCLS, 2003). The bacterial suspension was diluted 1:40 in sterile normal saline and served as the inoculum. Mueller-Hinton agar plates (hereafter referred to as MHA) were prepared by combining 16 g of agarose (Becton-Dickinson, Sparks, Md.), 22 g of Mueller-Hinton broth powder (Becton-Dickinson, Sparks, Md.), and adjusted to 1 L with distilled water. The agar was sterilized by autoclaving, cooled to 55° C., and supplemented with 25 μg/mL of glucose-6-phosphate (Sigma-Aldrich, St. Louis, Mo.). Twenty-five mL of cooled agar was aliquoted into 50 mL conical tubes and supplemented with appropriate concentrations of antibiotic to achieve concentrations ranging from 0.06 μg/mL to 512 μg/mL. After gently mixing the agar and antibiotic, the suspension was poured into sterile 100 mm petri dishes and allowed to solidify at room temperature. The antibiotic agar plates were inoculated with approximately 2×10⁴ CFU/spot with a 48-point inoculator (Sigma-Aldrich, St. Louis, Mo.). The MIC was defined as the lowest concentration of antibiotic (s) that prevented visible growth after incubation for 18-20 hours at 35° C. The efficacy of a particular antibiotic or antibiotic combination on large populations of P. aeruginosa was determined by calculating the MIC₅₀ and MIC₉₀ values. The MIC₅₀ value was defined as the concentration of antibiotic(s) which inhibited 50% of the P. aeruginosa strains. The MIC₉₀ value was defined as the concentration of antibiotic(s) which inhibited 90% of the P. aeruginosa strains (Wiedemann and Grimm, 1996).

Method B: The MICs of fosfomycin alone, tobramycin alone, or combinations of fosfomycin plus tobramycin were determined for P. aeruginosa strains in the presence of porcine gastric mucin to evaluate the effect of mucin and protein binding on antibiotic activity. Methodologies were identical to that described in Example 5, Method A, with the exception that 2% (weight/volume) porcine gastric mucin (Sigma Chemical Co., St. Louis, Mo.) was added to the MHA prior to autoclaving.

Method C: The MICs of amikacin, arbekacin, dibekacin, gentamicin, kanamycin, netilimicin, neomycin, streptomycin, and tobramycin alone were determined for P. aeruginosa ATCC 27853 by the broth-microdilution method according to NCCLS standards (NCCLS, 2003). E. coli ATCC 25922 and S. aureus ATCC 29213 were used as quality control stains. Bacterial strains were streaked onto blood agar plates, and incubated at 35° C. for 18 hours. Two to three bacterial colonies from the overnight cultures were inoculated into 3 mL of sterile normal saline, vortexed briefly, and adjusted to a 0.5 McFarland standard (NCCLS, 2003). The bacterial suspension was diluted 1:100 in cation-adjusted Mueller-Hinton broth (hereafter referred to as CAMHB). Fifty microliters of bacterial inoculum (approximately 2×10⁵ CFU/mL) was pipeted into individual wells of 96-well plates containing 50 μl of CAMHB (Remel, Lenexa, Kanas) supplemented with 2-fold dilutions of antibiotics ranging in concentration from 0.125 μg/mL to 128 μg/mL. The MIC was defined as the lowest concentration of antibiotic (s) that prevented visible growth after incubation at 35° C. for 18-24 hours.

EXAMPLE 6 Checkerboard Synergy

Potential interactions between fosfomycin and amikacin, arbekacin, dibekacin, gentamicin, kanamycin B, neomycin, netilimicin, streptomycin, or tobramycin were determined by the checkerboard method (Eliopoulos and Moellering, 1996). Two-fold serial dilutions of fosfomycin and aminoglycosides, which bracketed the expected MIC value for both compounds, were evaluated. Bacterial strains were streaked onto blood agar plates and incubated at 35° C. for 18-24 hours. Two to three bacterial colonies from the overnight cultures were inoculated into 3 mL of sterile normal saline, vortexed briefly, and adjusted to a 0.5 McFarland standard (NCCLS, 2003). Fifty microliters of bacterial inoculum (approximately 2×10⁵ CFU/mL) was pipeted into individual wells of 96-well plates containing 50 μl of CAMHB supplemented with 2-fold dilutions of the two antibiotics of interest. The fractional inhibitory concentration (FIC) was calculated as the MIC of compound # 1 in combination with a second compound, divided by the MIC of compound #1 alone. A summation of FIC (SFIC) was calculated for each drug combination as the sum of the individual FICs of compound #1 and #2. The FIC was calculated as the lowest concentration of antibiotics that that prevented visible growth after incubation at 18-20 hours at 35° C. Synergy was defined as an SFIC of ≦0.5, indifference as an SFIC >0.5 and <4, and antagonism as an SFIC ≦4. The lowest SFIC was used for final interpretation of drug interactions.

EXAMPLE 7 Determination of Time-Kill Kenetics

Time-kill experiments were performed in the presence of 2% porcine gastric mucin to evaluate the effect of mucin and protein binding on antibiotic activity. Two to three bacterial colonies were inoculated into 10 mL CAMHB and incubated at 35° C. in a shaking water bath (250 rpm) for 18-24 hours. A 1:40 dilution of the overnight culture was made in 10 mL of fresh CAMHB and incubated at 35° C. in a shaking water bath (250 rpm) for 1-2 hours. The resulting culture was adjusted to a 0.5 McFarland standard (NCCLS, 2003). To reduce variability in the bacterial inoculum size when comparing multiple antibiotics, one master tube of CAMHB containing 2% (weight/volume) of porcine gastric mucin was inoculated with a 1:200 dilution of bacterial inoculum (approximately 5×10⁵ CFU/mL), supplemented with 25 μg/mL of glucose-6-phosphate, and briefly vortexed. Ten milliliter aliquots were then pipeted into 50 mL conical tubes. Fosfomycin alone, tobramycin alone, and combinations of fosfomycin plus amikacin, arbekacin, dibikacin, gentamicin, kanamycin, netilimicin, neomycin, streptomycin, or tobramycin were added to the culture medium at concentrations equal to 1, 2, 4, and 8-fold multiples of the fosfofomycin MIC (4 μg/mL) for P. aeruginosa ATCC 27853. The killing activity of fosfomycin plus amikacin, arbekacin, dibikacin, gentamicin, kanamycin, netilimicin, neomycin, streptomycin, or tobramycin were also compared to the killing activity of their individual components alone. For example, 32 μg/mL of a 9:1 fosfomycin:tobramycin combination was compared to killing activity of 28.8 μg/mL of fosfomycin alone and 3.2 μg/mL of tobramycin alone. A no drug control was run in each experiment. Cultures were incubated with antibiotic at 35° C. in a shaking (250 rpm) water bath for 24 hours. Bacterial killing was determined at 0, 1, 2, 4, 6, and 24 hours by making 10-fold serial dilutions of the cultures in sterile normal saline, and spreading 100 μl aliquots on blood agar plates. Culture plates were incubated at 35° C. for 18-24 hours and the number of colonies enumerated manually. The limit of detection for the colony counting method was 1 Log₁₀. Time-kill curves were constructed by plotting the Log₁₀ number of CFU/mL culture versus time. Antibiotic concentrations that reduced the original inoculum by ≧3 Log₁₀ CFU/mL were considered bactericidal, and concentrations that reduced the original inoculum by ≦2 Log₁₀ CFU/mL were defined as bacteriostatic (NCCLS, 1999). Synergism was defined as a reduction of bacterial colony counts of ≧2 Log₁₀ CFU/mL with the antibiotic combinations compared to the most active single antibiotic (NCCLS, 1999).

EXAMPLE 8 Determination of Minimal Bactericidal Concentration (MBC)

The MBCs of fosfomycin, amikacin, arbekacin, dibikacin, gentamicin, kanamycin B, netilimicin, neomycin, streptomycin, or tobramycin alone for P. aeruginosa ATCC 27853, E. coli ATCC 25922, and S. aureus ATCC 29213 were determined by the broth microdilution method according to NCCLS standards (NCCLS, 1999). Bacterial strains were streaked onto blood agar plates and incubated at 35° C. for 18-24 hours. Two to three bacterial colonies from the overnight cultures were inoculated into 3 mL of sterile normal saline, vortexed briefly, and adjusted to a 0.5 McFarland standard (NCCLS, 2003). Fifty microliters of bacterial inoculum (approximately 2×10⁵ CFU/mL) was pipeted into individual wells of 96-well plates containg 50 μl of CAMHB (Remel, Lenexa, Kanas) supplemented with 2-fold dilutions of antibiotics ranging in concentration from 0.125 μg/mL to 128 μg/mL. Plates were incubated at 35° C. for 18-24 hours and MIC determined as described in Example 5, Method C. The contents of wells showing no growth (MIC and above) were mixed with a pipetor and duplicate 10 μl samples spread onto blood agar plates. Culture plates were incubated at 35° C. for 18-24 hours and the number of bacterial colonies on each plate enumerated manually. Rejection values were determined by NCCLS methods which considers the final inoculum size, single or double sampling, pipetting error, and the Poisson distributuion of sample responses (NCCLS, 1999). For example, if the final inoculum was 5×10⁵ CFU/mL and duplicate samples were evaluated, the lowest dilution having fewer than a total of 25 colonies was considered the MBC. The MBC was defined as the ≧3 Log₁₀ decrease in CFU/mL of the original inoculum as described by NCCLS standards (NCCLS, 1999). The MBC/MIC ratios were calculated by dividing the MBC by the MIC.

EXAMPLE 9 Determination of Frequency of Single-Step Resistance

The frequency of single-step spontaneous resistance mutation was determined for 5 susceptible P. aeruginosa strains. The bacterial inoculum was prepared by inoculating 5 mL of CAMHB with 2-3 bacterial colonies and incubating at 35° C. in a shaking water bath (250 rpm) for 18 h. A 1:20 dilution of the overnight culture was made into 50 mL of fresh CAMHB in a 125 ml erlenmyer flask and incubated at 35° C. in a shaking waterbath (250 rpm) for 8 hours. Cultures were centrifuged at 2,500 rpm, room temperature, for 20 min. The supernatant was decanted off and the cell pellet from the 50 mL of culture resuspended in 500 μl-1000 μl CAMHB. The CFU/mL in the bacterial suspension was determined by making 10-fold serial dilutions in sterile normal saline, and spreading 100 μl aliquots on blood agar plates. Culture plates were incubated at 35° C. for 18-24 hours and the number of colonies enumerated manually. One hundred microliters of bacterial cell suspension (approximately 10⁹ CFU) was spread onto MHA plates containing 128 μg/mL of fosfomycin, 8 μg/mL of tobramycin, or 128 μg/mL of fosfomycin plus 8 μg/mL of tobramycin. The culture plates were incubated at 35° C. for 48-72 hours and the number of colonies on each plate was enumerated manually. The frequency of resistance was calculated by dividing the number of bacteria growing at the defined antibiotic concentration by the number of bacteria in the original inoculum. Representative mutants were evaluated for changes in MIC to fosfomycin alone, tobramycin alone, and fosfomycin plus tobramycin combinations as described in Example 5, Method A.

EXAMPLE 10 Multistep Resistance Analysis

Development of resistance during continuous serial passage was evaluated with a clinical P. aeruginosa strain susceptible to fosfomycin (MIC=8 μg/mL) and tobramycin (MIC=0.5 μg/mL). A single colony of P. aeruginosa COR-014 was inoculated into 5 mL of MHCAB and briefly vortexed. Two-fold serial dilutions of fosfomycin alone covering the range of 1 μg/mL-512 μg/mL, tobramycin alone covering 0.0625 μg/mL-512 μg/mL, and a 9:1 fosfomycin:tobramycin combination covering 0.0625 μg/mL-512 μg/mL were made in CAMHB. Ten microliters of bacterial suspension was pipetted into 3 mL of each antibiotic dilution and incubated statically at 35° C. for 18-24 hours. Tubes with the highest concentration of antibiotic having visible bacterial growth were selected from each antibiotic dilution series and 100 μl of the culture was pipeted into a fresh antibiotic dilution series. Tubes were incubated statically at 35° C. for 18-24 hours. This process was repeated for a total of 28 times. MICs of fosfomycin alone, tobramycin alone, and fosfomycin plus tobramycin were determined for the parent strain and resistant isolates collected after each passage as described in Example 5, Method C.

EXAMPLE 11 Determination of Animal Efficacy

The in vivo efficacy of aerosolized fosfomycin alone, tobramycin alone and combinations of fosfomycin plus tobramycin were evaluated against E. coli ATCC 25922 and P. aeruginosa ATCC 27853 in rat lungs. The bacterial inoculum used to establish the rat lung infections consisted of a mixture of bacteria and agarose beads. E. coli ATCC 25922 or P. aeruginosa ATCC 27853 was streaked onto blood agar plates and incubated for 18-24 hours at 35° C. Two to three colonies were inoculated into 10 mL Tryptic Soy Broth (hereafter referred to as TSB) and incubated statically at 35° C. for 1.5 to 2 hours. The culture was adjusted to an optical density of 0.1 (625 nm) in fresh TSB and then diluted 1:5 in TSB. One milliliter of bacterial suspension (approximately 10⁷ CFU/mL) was added to 10 mL of a 2% noble agar solution and briefly mixed by inversion. The mixture was added to 150 mL of heavy white mineral oil equilibrated to 55° C. and stirred on ice for 5 minutes. The cooled suspension was poured into a 250 mL centrifuge tube and centrifuged at 3,000 rpm, 4° C., for 10 minutes. The supernatant was decanted off, the pellet resuspended in 25 mL of normal sterile saline, and transferred to a 50 mL conical centrifuge tube. Following centrifugation at 3,000 rpm, 4° C., for 10 minutes, the supernatant was decanted off and the pellet resuspended in 10 mL of saline. The suspension was centrifuged at 3,000 rpm, 4° C., for 10 minutes and the supernatant decanted off. The pellet was resuspended in approximately 10 mL sterile normal saline and then diluted 1:30 in a sterile 2% agarose bead solution to give approximately 10-1000 CFU/mL.

Prior to intratracheal installation of the bacterial inoculum, male Sprague-Dawly rats (200-250 g) were anesthetized by exposure to isoflurane for 5 minutes. An intratracheal needle was inserted into the trachea and 80 μl of agarose beads containing approximately 10-100 CFU of E. coli ATCC 25922 or P. aeruginosa ATCC 27853 were instilled into the lungs with a 1 mL syringe. The rats were placed into individual cages and allowed to recover for approximately 18 hours.

Fosfomycin alone, tobramycin alone, and fosfomycin:tobramycin combinations were administered to rats using an aerosol exposure device (In Tox Products, New Mexico). The system consisted of a central chamber having separate aerosol supply and exhaust paths. The central chamber had 24 ports that were directly connected to the aerosol supply system. Rats were placed into individual aerosol exposure tubes and restrained with an adjustable push plate and end cap assembly so they could not turn around or back away from the end of the tube. The restraint tubes containing the rats were loaded onto ports on the central chamber and the air flow adjusted to 1 liter/min. The air (breathable quality air) flow to the PARI LC Star nebulizer was held constant at 6.9 liters per minute. Fosfomycin alone, tobramycin alone, or 9:1, 8:2, or 7:3 fosfomycin:tobramycin combinations were nebulized at 0.2 mL/min and delivered to the rats via the aerosol supply path. Additional air (hereafter referred to as dilution air) was delivered to nebulizer (where) to balance the over pressure of the air used to deliver aerosolized drugs to the rats. Rodents were exposed to aerosolized antibiotics for up to 2 hours, twice daily for 3 consecutive days. Each treatment group consisted of 5-8 animals per group. A non-treatment control was included in each experiment.

Bacterial killing was evaluated 18 hours after the last exposure. Rats were anesthetized with isoflurane and euthanized by intraperitoneal administration of 500 μl of phenobarbitol. The lungs were removed aseptically, excess tissue removed and the lung weight determined. Lungs were placed in 10 mL glass vials, and 3 mL of normal sterile saline added per gram tissue. Samples were homogenized with hand-held homogenizer for 30 seconds. Bacterial killing was determined by making 10-fold serial dilutions of the lung homogenate in sterile normal saline, and spreading 100 μl aliquots on blood agar plates. Culture plates were incubated at 35° C. for 18-24 hours and the number of bacterial colonies enumerated manually. Antibiotic efficacy was determined by comparing the CFU's/lung from the non-treatment control group to the treatment groups.

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1. An aerosol formulation for the prevention and treatment of infection caused by susceptible bacteria in the respiratory tract of a patient said formulation comprising from about 1 mg to about 300 mg of fosfomycin, about 1 mg to about 300 mg of tobramycin, and about 0.5-7 mL of solution containing a chloride concentration >30 mM; wherein said formulation is to be administered by aerosolization using a nebulizer able to produce an aerosol with a predominant particle size distribution between 1 and 5 μm.
 2. The aerosol formulation of claim 1 wherein said formulation is packaged in a dual blow-fill seal ampoule such that each antibiotic is individually formulated in water and pH adjusted such that the fosfomycin solution has a pH between 8-13 and the tobramycin solution has a pH between 1-6 and, when the dual ampoule is opened and the contents reconstituted, the resulting solution has a pH between about 4.5 and 8.0.
 3. The aerosol formulation of claim 1 wherein the weight ratio of fosfomycin to tobramycin is from about 5 to about 9 parts fosfomysin to from about 1 part to about 5 parts tobramycin.
 4. The aerosol formulation of claim 1 wherein the weight ratio of fosfomycin to tobramycin is from about 7 to about 9 parts fosfomysin to from about 1 part to about 3 parts tobramycin.
 5. The aerosol formulation of claim 1 wherein the weight ratio of fosfomycin to tobramycin is about 8 parts fosfomysin to about 2 parts tobramycin.
 6. A method for the prevention and treatment of infections of the respiratory tract caused by bacteria comprising administering to a patient in need of such treatment an effective amount of the formulation of claim
 1. 7. The method of claim 6 wherein said formulation has a pH between about 4.5 and 8.0 in water with a chloride concentration >30 mM.
 8. The method of claim 6 wherein said formulation is administered by aerosolization using a jet, ultrasonic, pressurized or vibrating porous plate nebulizer or other device capable of delivering the formulation to the nasal passages or pulmonary airway and the aerosol has with a particle size distribution predominately between 1 and 5 μm.
 9. The method of claim 6 wherein the infections are P. aeruginosa or S. aureus.
 10. The method of claim 6 wherein the infections are selected from the group consisting of bacterial sinucitis, cystic fibrosis, pulmonary Pseudomonas, hospital and community associated pneumonia and ventilator associated pneumonia.
 11. An aerosol dry powder formulation for the prevention and treatment of infection caused by susceptible bacteria in respiratory tract of a patient said formulation comprising from about 1 mg to about 300 mg of fosfomycin, about 1 mg to about 300 mg tobramycin, and pharmaceutically acceptable excipients said formulation having a particle size between 1-5μ; and said formulation adapted to be administered to the pulmonary airway by a dry powder inhaler.
 12. A method for the prevention and treatment of infections of the respiratory tract caused by the bacteria P. aeruginosa comprising administering to a patient in need of such treatment an aerosol formulation comprising the formulation of claim
 13. 13. A reconstituted solution of about 1 to about 300 mg of fosfomycin and about 1 to about 300 mg tobramycin suspended in about 0.5-7 mL of a physiologically suitable solution. 